Most industrial waters contain alkaline earth metal cations such as calcium, barium, magnesium, iron, manganese, zinc, etc., and several anions such as bicarbonate, carbonate, sulfate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until these product solubility concentrations are no longer exceeded. For example, when the ionic product of calcium and carbonate exceeds the solubility of the calcium carbonate, a solid phase of calcium carbonate will form.
Solubility product concentrations are exceeded for various reasons such as partial evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on surfaces of the water carrying system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. This scale is an expensive problem in many industrial water systems, such as recirculating cooling water systems in cooling towers, in that the scale causes delays and shutdowns for cleaning and removal of the scale.
Fouling of heat transfer systems in industrial water systems and fouling of semipermeable membranes in reverse osmosis systems by mineral scales is a constant threat to the efficient operation of such systems. Examples of mineral scales contemplated herein include calcium carbonate, calcium sulfate, calcium phosphate, iron oxides, iron silicates and the like, whereas examples of industrial water systems include cooling and boiler water, desalination, oil production and the like. The various types of scale control agents in such systems include sodium hexametaphosphate, phosphonates, polyacrylates, copolymers of acrylic acid and methacrylic acid, copolymers of acrylic acid and acrylamide, and the like.
It is known that trace amounts of soluble iron when present in a water system can give rise to the formation of insoluble iron oxide and/or iron silicate scale and can adversely affect the performance of scale control agents. Soluble iron, which is present in the feed water, can be stabilized by the use of chelating agents such as citric acid and gluconic acid but such chemicals are not good scale control agents.